The present invention relates to a gravity compensating mechanism for an articulated type industrial robot.
The apparatus of the prior art will be described in the following with reference to FIG. 3. In the known direct drive type articulated industrial robot, second and third motors are mounted on a swivel 32 which is driven to whirl in a horizontal plane by a first motor mounted on a stationary base 31. This industrial robot includes: a first lower arm (i.e., first arm) 34 supported to whirl on a first horizontal axis 35 and driven to whirl in the direction of gravity by the second motor; a second lower arm (i.e., second arm) 38 supported to whirl on a second horizontal axis 36 at the upper end of the first arm 34 and driven to whirl in the direction of gravity by the third motor; and an upper working arm (i.e., third arm) 37 connected to the second arm 38 through a link 37a to whirl in the direction of gravity.
In the direct drive type articulated industrial robot of this type, the moment on the first horizontal axis 35 caused by the gravities of the third arm 37 and the first arm 34 are applied to the second motor for driving the first arm 34, and the moment on the second horizontal axis caused by the gravity of the third arm 37 is applied to the third motor for driving the third arm 37. Since these moments will change with the angles of gradient of the first arm 34 and the third arm 37, the magnitude of the loads to be applied to the second motor and the third motor change with the angles of gradient of the first and third arms.
The moment due to the gravity will take its maximum, when the arm is in a horizontal position, and will decrease along the COS curve, as the arm whirls, until it takes a zero value when the leading end of the arm is directed vertically upward or downward. In order to generate a moment in the opposite direction to balance the above-specified moment, the robot is equipped in the prior art with a gravity compensating mechanism.
In the gravity compensating mechanism of the prior art, as shown in FIG. 3, the gravity is compensated by a spring mechanism 33 which has its one and other ends attached to the swivel 32 and an upper portion of the first arm 34.
In the case of this gravity compensating mechanism of the prior art, the moment resulting from the force of the spring mechanism 33 in the direction opposite to the moment on the first horizontal axis 34 due to the gravity is given on the first horizontal axis 35. Despite this fact, however, there arise problems: the whirling range of the first arm 34 is restricted by the spring mechanism 33; the gravity compensating mechanism of the spring mechanism 33 is disposed outside of the first arm 34 to restrict the working space; the moment of the third arm 37 on the second horizontal axis 36 due to the gravity cannot be compensated so that the third motor is large-sized to compensate the moment on the second horizontal axis.